Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
Command-Shift-4, then select an area: Take a screenshot of an area and save it as a file on the desktop
Command-Shift-4, then space, then click a window: Take a screenshot of a window and save it as a file on the desktop
Command-Control-Shift-3: Take a screenshot of the screen, and save it to the clipboard
Command-Control-Shift-4, then select an area: Take a screenshot of an area and save it to the clipboard
Command-Control-Shift-4, then space, then click a window: Take a screenshot of a window and save it to the clipboard

In Leopard and later, the following keys can be held down while selecting an area (via Command-Shift-4 or Command-Control-Shift-4):

Space, to lock the size of the selected region and instead move it when the mouse moves
Shift, to resize only one edge of the selected region
Option, to resize the selected region with its center as the anchor point.
But unfortunately, all above has no tips for capturing irregular on Mac, I think you can Onde Screen capture for Mac(Screen Capture for Mac,screen grabbing utility,capture screen shot on Mac.). It is a new-released capturing software for Mac users.